Laboratory for Soil and Water Management, Katholieke Universiteit Leuven, 3001 Heverlee, Belgium.
Plant Physiol. 2012 Oct;160(2):1097-109. doi: 10.1104/pp.112.202200. Epub 2012 Aug 3.
It has long been recognized that diffusive boundary layers affect the determination of active transport parameters, but this has been largely overlooked in plant physiological research. We studied the short-term uptake of cadmium (Cd), zinc (Zn), and nickel (Ni) by spinach (Spinacia oleracea) and tomato (Lycopersicon esculentum) in solutions with or without metal complexes. At same free ion concentration, the presence of complexes, which enhance the diffusion flux, increased the uptake of Cd and Zn, whereas Ni uptake was unaffected. Competition effects of protons on Cd and Zn uptake were observed only at a very large degree of buffering, while competition of magnesium ions on Ni uptake was observed even in unbuffered solutions. These results strongly suggest that uptake of Cd and Zn is limited by diffusion of the free ion to the roots, except at very high degree of solution buffering, whereas Ni uptake is generally internalization limited. All results could be well described by a model that combined a diffusion equation with a competitive Michaelis-Menten equation. Direct uptake of the complex was estimated to be a major contribution only at millimolar concentrations of the complex or at very large ratios of complex to free ion concentration. The true K(m) for uptake of Cd(2+) and Zn(2+) was estimated at <5 nm, three orders of magnitude smaller than the K(m) measured in unbuffered solutions. Published Michaelis constants for plant uptake of Cd and Zn likely strongly overestimate physiological ones and should not be interpreted as an indicator of transporter affinity.
长期以来，人们已经认识到扩散边界层会影响主动运输参数的确定，但这在植物生理学研究中基本上被忽视了。我们研究了在有或没有金属配合物的溶液中，菠菜（Spinacia oleracea）和番茄（Lycopersicon esculentum）对镉（Cd）、锌（Zn）和镍（Ni）的短期吸收。在相同的游离离子浓度下，增强扩散通量的配合物的存在增加了 Cd 和 Zn 的吸收，而 Ni 的吸收则不受影响。只有在很大的缓冲程度下，质子对 Cd 和 Zn 吸收的竞争效应才会显现出来，而即使在没有缓冲的溶液中，镁离子对 Ni 吸收的竞争效应也会显现出来。这些结果强烈表明，除了在非常高的溶液缓冲度下，Cd 和 Zn 的吸收受到自由离子向根部扩散的限制，而 Ni 的吸收通常受到内化的限制。一个将扩散方程与竞争米氏方程相结合的模型可以很好地描述所有结果。只有在配合物的浓度达到毫摩尔级别或配合物与游离离子浓度之比非常大的情况下，配合物的直接吸收才被估计为主要贡献。Cd(2+)和 Zn(2+)吸收的真实 K(m)估计值小于 5nm，比未缓冲溶液中测量的值小三个数量级。已发表的植物对 Cd 和 Zn 吸收的米氏常数可能严重高估了生理常数，不应将其解释为转运蛋白亲和力的指标。
